One class of position determination systems and devices determine the location of receivers using data broadcast by satellites. One constellation of satellites is the Global Positioning System (GPS). The GPS consists of a constellation of 24 orbiting satellites that transmit timing information and the satellite's ephemerides via microwave radio.
Position determination devices determine position by receiving the timing signals and ephemerides from four or more satellites. The timing information from each satellite is analyzed in order to determine the apparent distance from the position determination system to each satellite. The determination of apparent distance is made by measuring the time it takes for the signals to travel from each satellite to the receiver of the position determination system. These apparent distances are referred to as pseudoranges.
Pseudoranges are calculated by measuring the time it takes for the signal to travel from the satellite to the receiver. The satellites mark their transmissions digitally and the receiver compares the time it receives the time mark with its own time clock. The time delay, referred to as transit time, is typically in the range of about 70-90 milliseconds. Distance is then determined by multiplying transit time by the speed of radio transmissions (approximately 300,000,000 meters/second).
Since the ephemeride data includes the location of each satellite, position may be determined by a geometric calculation that uses the known satellite positions and calculated distances (pseudoranges). GPS based positions are calculated using the World Geodetic System of 1984 (WGS84) coordinate system. These positions are expressed in Earth Centered Earth Fixed (ECEF) coordinates of X, Y, and Z axes. These positions are often transformed into latitude, longitude, and height relative to the WGS84 ellipsoid.
One factor that introduces error into the process of determining location is atmospheric conditions. Another source of error results from the intentional introduction of error into the transmitted ephemerides and clock by the U.S. Air Force (referred to hereinafter as "selective availability" or "S/A"). The GPS navigation signals commonly available to civilian users are referred to as the standard positioning service (SPS). The accuracy of SPS is currently specified by the Department of Defense (DOD) to be within 100 meters horizontal position 95 percent of the time and 300 meters 99.99 percent of the time. Errors also result from atmospheric conditions. Though the specified horizontal accuracy may be adequate for some applications such as navigation of a vessel in the open ocean, other applications require an increased level of accuracy.
One method for obtaining accurate position that compensates for intentionally induced error and error due to atmospheric conditions is known as Differential GPS (DGPS). DGPS systems receive correction data broadcast from a DGPS reference station. A DGPS reference station is located at a fixed and known location. By using this information combined with the satellites' broadcast ephemerides, an actual range to each satellite is able to be determined. By differencing the received range measurement (pseudorange) with this calculated range, a correction to the pseudoranges received at other GPS receivers in the local area can be broadcast to those other receivers that are attempting to solve for their own local location. This correction includes all induced satellite clock errors and atmospheric (ionosphere, troposphere) errors.
DGPS systems typically determine position in one of two ways. Traditionally, positions have been calculated using code phase differential techniques. These are normally referred to as DGPS. More recently, carrier phase techniques have been used to determine position. These systems are referred to as Real Time Kinematic (RTK) systems.
DGPS reference stations may be dedicated facilities with permanent and/or extensive broadcast capabilities or may be simply a transient DGPS receiver with data transmitter located at a known location. DGPS reference stations transmit either their calculated corrections to the GPS signals or their actual observations of the GPS signals (raw data), or both. When transmitting calculated corrections, errors due to atmospheric (troposphere, ionosphere, etc.) and errors due to satellite timing/clock (both intentional and process noise) are represented by the correction value. The application of these corrections at a DGPS receiver will compensate for these error sources.
Differential GPS reference stations may also transmit their observations of the GPS signals for each satellite. This method of transmission is popular with RTK positioning techniques and systems due to the nature of typical RTK processing methods. When using this type of data format, errors associated with atmospherics and satellite timing/clock errors are removed at the moving/roving/differential GPS receiver. Most manufacturers of RTK systems typically broadcast this data in a format unique to the particular manufacture.
Other sources of correction data that include correction data for S/A and atmospheric conditions include broadcasts that conform to the Radio Technical Commission for Maritime services (RTCM) format. The RTCM has established standards describing format standards, communication bands, and messages for a differential correction GPS service. Correction data that conforms to the RTCM format is broadcast by the US Coast Guard and others to assist in maritime navigation. The US Coast Guard has regional DGPS reference stations that calculate and broadcast correction data using the RTCM format. The RTCM correction data broadcast by some US Coast Guard DGPS reference stations includes carrier phase observable data while data broadcast by other facilities only includes code phase correction data. However, irrespective of whether the particular US Coast Guard facility broadcasts carrier phase data or code phase correction data, the broadcast is typically in a standard RTCM format. Other agencies and port authorities throughout the world broadcast differential correction signals conforming to the RTCM format for navigation in and around coastal areas. Both raw observable data and RTCM "correction data" are referred to hereinafter as "correction data" since both forms of data allow for correction to be made to position.
FIG. 1 shows a prior art position determination system 10 for determining position using correction data originating from a DGPS Reference Station that transmits in a RTCM format. Position determination system 10 is shown to include housing 17 that contains beacon antenna 11 and beacon receiver 13. Housing 18 is shown to include GPS antenna 12 and GPS receiver 14. Both housing 17 and housing 18 are coupled to a third housing which contains DGPS processor 19 by electrical cable. Battery 15 is connected by electrical cable to DGPS processor 19 for providing electrical power to the components of position determination system 10. Data logger 16 is also shown to be coupled via electrical cable to DGPS processor 19. Data logger 16 typically includes a display and function keys so as to allow users to view output and to input data as required for the operation of position determination system 10. In operation, beacon antenna 11 receives differential correction signals from a Reference Station that broadcasts in a RTCM format and couples the signals to beacon receiver 13. Beacon receiver 13 demodulates the RTCM signals so as to obtain correction data which is then coupled to DGPS processor 19. GPS antenna 12 receives signals from satellites of the GPS and couples the signals to GPS receiver 14. GPS receiver 14 demodulates the signals from GPS satellites and processes the incoming data which is then coupled via electrical cable to DGPS processor 19. DGPS processor 19 then uses the data from beacon receiver 13 and GPS receiver 14 to accurately determine position.
Prior art position determination systems that use bulky housings that are connected via cable are difficult to transport and use. In addition, in many instances differential correction signals are not available or are not needed because a high level of accuracy is not required for a particular task. In these instances, a user must carry all of the different housings and components from place to place even though some of the features of the system are not required. In addition, different antennas and receivers are required for picking up the various different sources of correction data. For example, a position determination system having a receiver and antenna tuned to receive signals conforming to a particular manufacturer's format is used for determining position using correction data transmitted in a particular manufacturer's format. A separate position determination system that has a receiver and antenna tuned to receive RTCM signals is used to determine position using RTCM correction data.
One proposed new system for correcting position determination signals from satellites is the Wide Area Augmentation System (WAAS). The WAAS is designed for use with aircraft operations. The WAAS is designed to provide a system that has sufficient integrity such that position may be determined with sufficient reliability and accuracy for aircraft operations. The WAAS includes satellites for transmitting signals and a ground network that augments GPS such that GPS may be used as a primary navigation sensor for aircraft. The WAAS augments GPS with a ranging function, (which improves availability and reliability), differential GPS corrections (which improve accuracy), and integrity monitoring (which improve safety).
Prior Art FIG. 2 shows a proposed WAAS that includes WAAS satellite 4 that broadcasts GPS integrity and correction data, and a ranging signal that augments GPS. The WAAS ranging signal is GPS-like and may be received by slightly modified GPS receivers. More specifically, it will be at the GPS L1 frequency and will be modulated with a spread spectrum code from the same family as the GPS C/A codes. The code phase and carrier frequency of the signal is controlled so that the WAAS satellite will provide additional range measurements to a GPS user. The WAAS signal will also carry data that contains differential corrections and integrity information for all GPS satellites, as well as for the geostationary WAAS satellite 4.
The ground network shown in FIG. 2 accumulates differential corrections and integrity data at wide area Reference Stations (WRS) 2 that are widely dispersed. WRS 2 process the raw data received from GPS satellites to determine integrity, differential corrections, residual errors, and ionospheric delay information for each monitored satellite. They also develop ephemeris and clock information for the WAAS geostationary satellite 4. All of this data is accumulated at Wide area Master Site (WMS) 3 and is packaged into the WAAS message that is uplinked to the WAAS geostationary satellite 4 that broadcasts the WAAS signal. Aircraft such as aircraft 5 receive signals from GPS satellites such as GPS satellite 1 and receive the WAAS signal that then allows for accurately determining the position of aircraft 5. The WAAS signal does not interfere with GPS signals because the received WAAS signal has approximately the same power as GPS signals, and Code Division Multiple Access (CDMA) is used to share the L1 channel. In addition, position determination devices that use the WAAS do not need an additional antenna and receiver since the GPS antenna and receiver are used to pick up the WAAS signal.
However, prior art systems are designed either to receive and process WAAS signals (on the existing L1 receiver of the GPS position determination device), or to receive and process RTCM signals (using a radio receiver operating in the 300 KHz range), or receive and process correction data in a particular manufacturer's format (typically at a frequency in the unlicensed frequency band). Thus, prior art systems that use a particular manufacturer's format are not RTCM compatible. That is, they cannot use RTCM signals for accurately determining position. In addition, systems that are designed to receive and process WAAS signals are not RTCM compatible (they cannot use RTCM signals for accurately determining position).
What is needed is a position determination system that is easily moved from place to place, that is easy to use, and can use RTCM correction data when it is available and when it is required for accurately determining position. Also, what is needed is a way to obtain correction data from RTCM signals on a position determination device that is designed to receive correction data on unlicensed radio frequencies. In addition, a way to obtain correction data using RTCM signals on a position determination device that is designed to receive correction data in a format unique to a particular manufacturer is needed. In addition, a way to obtain correction data using RTCM signals on a position determination device that is designed to receive WAAS signals is needed. Furthermore, a position determination device that is easy to use and operate is required.